The International Symbiosis Society (ISS) Congress, held every third year, is a forum to communicate recent advances in the field of beneficial host-microbe interactions. The 5th Congress, held in Vienna, Austria in August 2006, brought together ~270 scientists from all over the world, and proceedings were published in a special issue of the journal Symbiosis. Continuing this exciting tradition, the 6th ISS Congress, to be held in Madison, Wisconsin August 9-15, 2009, will emphasize cutting-edge research on the evolution, ecology, metabolism, cellular biology, and molecular mechanisms of microbial symbiosis. The organizing committee has selected symposium themes that will highlight research showing the pervasive and profound beneficial contributions of microbes to host physiology, development, and health. Such symbiosis research supports an emerging paradigm that microbial communities within hosts, such as those in the mouth or intestine, are the functional equivalent of 'organs'that communicate and cooperate with other parts of the body. For example, microbial processing of ingested material produces compounds that are transported directly from the intestine to the liver, which in turn sends metabolites to the intestine by way of the bile. Through this network, the microbial community has a direct impact on all aspects of host physiology, including weight, nutrition, behavior, and efficacy of pharmaceutical treatments (Holmes et al. 2008, Nature 453:396). The 6th ISS Congress program has been designed to promote inter-disciplinary discussions of the mechanisms and consequences of microbial symbiosis, as well as novel approaches to study this important facet of human health. Research presentations have been arranged to spur transfer of ideas from the microscopic to the macroscopic scale, and from model systems to practical applications. This approach is particularly important as: 7 The interdisciplinary exchange of ideas that will occur at this conference is critical to harnessing and applying advances made in symbiosis research to the improvement of human health. 7 The ISS provides a synergistic platform on which researchers dedicated to elucidating the contributions of microbial communities to human health can interface with those investigating fundamental aspects of the evolution and ecology of symbiosis. 7 Microbial symbiosis is an intellectually stimulating subject that attracts pioneering graduate students and post-doctoral fellows who will benefit from participation in a dynamic and interdisciplinary conference. PUBLIC HEALTH RELEVANCE: Scientists have long recognized that microbes are intimately associated with plants and animals and can perform specific beneficial functions such as acquiring nutrients from the environment. More recently, research has demonstrated that like other animals, humans are profoundly affected by the microbial communities that live within them. Indeed, microbes influence human nutrition, weight, immunity, and responses to pharmaceuticals. The 6th International Symbiosis Society Congress is dedicated to promoting the emergence of new discoveries that improve human health by providing a forum to discuss experimental and theoretical advances in the study of beneficial microbes.